In extractive distillation (ED) and liquid-liquid extraction (LLE) processes for aromatics recovery, the solvent is circulated in a closed loop indefinitely. The feedstock is typically treated in a prefractionator to remove the heavy portion before being fed into the EDC or LLE column. Nevertheless, measurable amounts of heavy hydrocarbons (HCs) pass through even a well-designed prefractionator operating under normal conditions. The level of heavy HCs in the feed stream is significantly higher for a poorly operated or malfunctioned prefractionator. To remove the heavy HCs and the polymerized heavy materials derivate from oxidized solvent, conventional commercial LLE processes use a thermal solvent regenerator where a small slip stream of the lean solvent is heated to recover the regenerated solvent and heavy components that have boiling points lower than that of the solvent. The heavy polymeric materials (PMs), that have boiling points higher than that of the solvent, are removed as sludge from the bottom of the solvent regenerator.
U.S. Pat. No. 4,048,062 to Asselin discloses a LLE process for aromatics recovery in which a portion of lean solvent from the bottom of a solvent recovery column (SRC) is introduced into a solvent regenerator (SRG). A stripping steam that is introduced into the SRG separately is recovered with the regenerated solvent and then introduced into the SRC as a portion of the total stripping steam. This solvent regeneration scheme works because, within the same type of molecules, the higher the boiling point, the lower the polarity (affinity with the extractive solvent). Consequently, a major portion of the measurable heavy (C9 to C12) HCs in the feedstock is rejected by the solvent phase in the LLE column and is removed with the raffinate phase as a part of the non-aromatic product.
In an ED process for aromatics recovery, the heavy HCs tend to remain in the rich solvent at the bottom of the extractive distillation column (EDC) due to their high boiling points. Even for a narrow boiling-range (C6-C7) feedstock, there can be measurable amounts of heavy (C9+) HCs that are trapped and accumulated in the solvent, which can only be removed from the solvent by increasing the severity of the SRC (higher temperature and vacuum level, and more stripping steam) and/or by increasing the loading of the SRG. Neither alternative is desirable. Moreover, for the full boiling-range (C6-C8) feed, the boiling points of the heavy HCs are too high to be stripped from the solvent in the SRC and, as a result, they accumulated in the solvent as the solvent is circulated between the EDC and the SRC indefinitely in a closed loop.
The solvent regeneration of the Asselin scheme is not suitable for the ED process. The scheme was designed for LLE processes to remove minor amounts of PMs generated from reactions between the oxidized or decomposed solvent components and traces of the heavy HCs in the solvent. When this scheme is applied to ED processes, heavy HCs inevitably accumulated and polymerized in the closed solvent loop until the polymerized materials reach boiling points that are higher that of sulfolane (>285° C.) before they can be removed from the bottom of the solvent regenerator. This accumulation is potentially disastrous since the presence of excessive PMs not only changes the solvent properties (selectivity and solvency) significantly but the polymers also plug process equipment to render the ED process inoperable.
U.S. Pat. No. 7,666,299 to Wu, et al. and U.S. Pat. No. 7,871,514 to Lee, et al. disclose a technique for removing heavies from solvent that is based on the observation that most of extractive solvents for ED and LLE are water soluble. In practice, a split solvent stream is introduced into a water washing zone and contacts a stream of process water, which is circulated in a closed loop. Solvent dissolves into the water phase while the heavy HCs and PMs are rejected by the water. In this fashion, the heavy HCs and PMs are removed from the solvent stream and accumulate in the HC phase. Because this water wash method requires much water, it is often difficult to achieve the proper balance and distribution of the process water in the closed system.
U.S. Pat. No. 8,246,815 to Wu, et al. describes a method of removing heavy HCs and PMs that are trapped in the closed solvent loop in an ED or LLE process for aromatic HCs recovery. Light hydrocarbons, such as non-aromatic HCs in the raffinate stream, function as “displacement agents.” The light HCs “squeeze” the heavy HCs and PMs from the extractive solvent, especially when the heavy HCs in the solvent are in the C9+ molecular weight range.